The invention relates to a method of forming thin layer devices from superconducting materials on a substrate. 1t relates to thin layer superconducting devices.
The invention relates more particularly to a method of preparing thin layers of oxides which are superconducting at a high critical temperature. The superconducting properties of these compounds being intimately dependent on the crystalline structure, this is obtained directly during epitaxial growth on an appropriate substrate.
Superconducting oxides are revolutionizing the field of superconductivity and its applications. The first reference thereto was made by J. G. Bednorz and K. Muller in the document Z. Phys. B 64, 1986, pages 189 to 193 relating to a barium doped compound LA-Cu-O in which the beginning of superconducting transition is observed at about 35K. Doping with strontium raises this temperature slightly and makes this superconductivity more massive. Finally, the substitution of yttrium tor lanthanum gives rise to a critical temperature of 93.degree. kelvin, as was reported by M. K. Wu et al in the document Physical. Review. Letters 58, 1987, pages 908 to 910.
The existence of materials which are superconducting at temperatures higher than that of liquid nitrogen (77K) makes it possible to do away with the need for working in liquid helium, a requirement which greatly limited up to now the development of superconductivity applications. Among these latter, a large proportion (junction devices, IR absorbents, memories, . . . ) must be formed from thin layers. The preparation of the superconducting material in this form is thus essential.
All of the known superconducting oxides have a narrow correlation between the superconducting properties and the crystallographic structure. This is always true for the perovskite type. In the La.sub.2-x Ba.sub.x CuO.sub.4-y and La.sub.2-x Sr.sub.x CuO.sub.4-y systems, the structure is of K.sub.2 NiF.sub.4 type, as described for example by N. Nguyen, F. Studer and B. Raveau in the document Journal. Physical. Chemical. Solids, 44, 1983, pages 389 to 400. In the YBa.sub.2 Cu.sub.3 O.sub.7-y system, the structure is slightly different but still of perovskite type, as is reported for exampIe by T. Siegrist et al, Physic Review B 35, 1987, pages 7137 to 7139.
Apart from the structure, the other fundamental parameters for obtaining superconducting properties in these systems is the oxygen content, as described for example in the article by R. J. Cava et Coll, phys. Rev. Lett, 58, 1987, pages 408 to 410.
The whole of the results reporled up to now relate to materials prepared in the form of sintered powders, which may be described as a juxtaposition of small crystalline grains. The appropriate oxygen proportion is obtained by annealing in an oxygen atmosphere. The invention relates to the preparation of these materials in the form of thin layers epitaxied on substrates chosen so that the structure corresponding to the superconducting properties will be obtained.